Ignition devices used for hooking up blasting salvo networks commonly include signalling means in the form of electrical wires, fuses or detonating cords in addition to activation means such as caps or detonators. The surface parts of the network in particular are vulnerable to damages before and during system activation. Ignition failures may result in explosive and sharp detonators remaining in the blasted rock mass. Signalling means of pyrotechnic nature poses special safety problems on the design as explosive system components make the network potentially self-destructive. The full lengths of detonating cords must be carefully positioned and connected to avoid self-cutting or damage on neighboring lines. Low energy fuses are commonly ended with explosive transmission or delay caps which may be destructive both by direct explosive shock and by the action of splinters from its metal parts. Blocks and fixtures are frequently used to securely enclose, position and in particular to connect and branch the various network parts. Block connections may be used both purely between fuses, such as between a primary detonating cord and secondary cords or fuses, and between transmission or delay caps and outgoing fuses or cords. Properly designed, connecting blocks may reduce the destructive action of explosives by aligning fuses, by absorbing or directing the explosive shock and by catching splinters. Improperly designed, the blocks may add to the problems by misaligning the components, by amplifying the explosion through confinement and by itself being the source of shrapnel parts.
Hence, suitable block characteristics means partly contradictory requirements. Although a limited explosive action is desirable to avoid the problems outlined, a sufficiently strong action is needed to secure signal transmission to the secondary ignition devices, which for safety reasons always are made insensitive. Good construction against internal damage must not interfere with maintained good properties against external damage from vehicles, falling rocks etc. on the site. Practical considerations place further constraints on the design. The device is used once which excludes massive, elaborate or otherwise expensive constructions. Connections are made in the field and any feature conditioned by safety must still allow connections to be made easily, reproducibly and without extensive requirements for skill. In manufacture the block must allow rational manual or machine assemby with various kinds of permanent ignition devices.